This invention relates to a process for the regeneration of adsorbers loaded with impurities removed from the compressed air, and in particular to a process wherein regeneration is conducted with a hot regeneration gas.
In air rectification the air is compressed, then freed of impurities, particularly water vapor, carbon dioxide, acetylene, and other hydrocarbons in adsorbers and then cooled, partially liquified, and fed to a rectification column. There, the air is separated into a gaseous fraction, essentially containing nitrogen, and a liquid fraction, essentially containing oxygen.
Compression of the air usually is effected by one- or multi-stage compression to a pressure of 5 to 10 bar, typically 5 to 6 bar if nitrogen is withdrawn at about atmospheric pressure from the rectification plant and correspondingly higher if high pressure products are to be produced. As the efficiency of the compressor decreases with increasing compression ratio and, on the other hand, the efficiency of the compressor is an essential item in the overall energy balance of the process, the air usually is compressed under conditions that the compression ratio in each stage of compression is relatively low. For example, compression of ambient air to about 5 to 6 bar may be effected in a three- or four-stage turbo compressor. As a consequence of the relatively low compression ratio, the temperature of the air after each compression stage is relatively low, typically about 80.degree. to 130.degree. C., especially 95.degree. to 125.degree. C.
The adsorbers, typically molecular sieve adsorbers, generally operated by an alternating switching arrangement, are sequentially loaded with impurities from the air and then purified by regeneration gas. Preferably, a separation product, especially nitrogen, from the rectification column is used as the regeneration gas (see, e.g., DE-OS 30 12 062, incorporated by reference herein). The nitrogen is heated to about ambient temperature and divided into two partial streams, one being used as regeneration gas and the other one being withdrawn from the process and used for other purposes. The regeneration gas, after being further heated in an electric heater, is passed through one of the adsorbers to remove the impurities and is withdrawn from the plant as waste gas.
In a subsequent step of the regeneration phase, the heated adsorber is again cooled. For this purpose, the regeneration gas, without being heated, is fed directly to the adsorber, thereby cooling same, and is withdrawn from the adsorber as waste gas. For the purpose of regeneration, usually only a limited amount of regeneration gas is available, typically about 15 to 30% of the low pressure nitrogen.
For the regeneration of the adsorbers, the required high temperature for the regeneration gas has involved the use of heat exchangers having very large heating surfaces, resulting in large pressure drops and requiring a considerable expenditure of energy.
In a known process, the regeneration gas temperature is in the order of 150.degree. to 300.degree. C. In this process, which is disclosed in U.S. Pat. No. 4,329,158 of SIRCAR, the regeneration gas is heated by indirect heat exchange with compressed feed air having a temperature of 150.degree. to 300.degree. C. The whole amount of waste gas from an air separation plant, i.e. the whole nitrogen content of the air, is used as regeneration gas in this process. It is obvious that regeneration of an adsorber is easy in case of such high regeneration gas temperature and such large amounts of regeneration gas.